/* Problem URL: https://codeforces.com/problemset/problem/919/D */

#include <bits/stdc++.h>

using namespace std;

#define V vector

#define rmin(a, b) a = min(a, b)
#define rmax(a, b) a = max(a, b)

#define rep(i, lim) for (size_t i = 0; i < (lim); i++)
#define nrep(i, s, lim) for (size_t i = s; i < (lim); i++)

#define repv(i, v) for (auto &i : (v))
#define fillv(v) for (auto &itr_ : (v)) { cin >> itr_; }
#define sortv(v) sort(v.begin(), v.end())
#define all(v) (v).begin(), (v).end()

using vi = vector<int>;
using vvi = vector<vi>;
using vvvi = vector<vvi>;
using vvvvi = vector<vvvi>;

using ll = long long;

using vl = vector<ll>;
using vvl = vector<vl>;
using vvvl = vector<vvl>;
using vvvvl = vector<vvvl>;

template<class v>
auto operator<<(ostream &os, const vector<v> &vec)->ostream& {
    os << vec[0];
    for (size_t i = 1; i < vec.size(); i++) {
        os << ' ' << vec[i];
    }
    os << '\n';
    return os;
}

template<class v>
auto operator>>(istream &is, vector<v> &vec)->istream& {
    for (auto &i : vec) {
        is >> i;
    }
    return is;
}

template<class v>
auto operator<<(ostream &os, const vector<vector<v>> &vec)->ostream& {
    for (auto &i : vec) {
        os << i[0];
        for (size_t j = 1; j < i.size(); j++) {
            os << ' ' << i[j];
        }
        os << '\n';
    }
    return os;
}

template<class v>
auto operator>>(istream  &is, vector<vector<v>> &vec)->istream& {
    for (auto &i : vec) {
        for (auto &j : i) {
            is >> j;
        }
    }
    return is;
}

int main()
{
    ios::sync_with_stdio(false);
    cin.tie(nullptr);

    int n, m;
    cin >> n >> m;

    string a;
    cin >> a;

    vvi graph(n);
    while (m--) {
        int u, v;
        cin >> u >> v;
        u--, v--;

        graph[u].push_back(v);
    }

    vvi dp(n, vi(26, 0));
    V<bool> vis(n, false);
    V<bool> current(n, false);

    function<bool(int i)> dfs = [&](int i){
        vis[i] = true;
        current[i] = true;

        for (auto j : graph[i]) {
            if (current[j]) {
                return true;
            }

            if (!vis[j]) {
                if (dfs(j)) {
                    return true;
                }
            }

            rep(k, 26) {
                rmax(dp[i][k], dp[j][k]);
            }
        }

        dp[i][a[i] - 'a']++;
        current[i] = false;
        return false;
    };

    int ans = 0;
    rep(i, n) {
        if (!vis[i]) {
            if (dfs(i)) {
                cout << "-1\n";
                return 0;
            }
            rmax(ans, *max_element(all(dp[i])));
        }
    }

    cout << ans << '\n';
}